1. FIELD OF THE INVENTION
The present invention relates to a recording disc assembly having a magnetic recording disc or optical recording disc.
2. DESCRIPTION OF THE PRIOR ART
In a recording disc assembly of the above type, there is known a recording disc assembly as shown in FIG. 1, wherein 1 denotes a case made of rigid materials, such as metal or plastic resin, 2 denotes a hub made of plastic resin materials, 2' an upper hub, 3 an annular supporting plate, 4 a ring like stopper and 5 a recording medium, such as a magnetic recording disc.
Referring to FIG. 2 there is shown a top plan view of the annular supporting plate 3, wherein a through hole 3a is defined in the central portion of the plate 3 with the hub 2 secured to the central portion of the plate 3 by an outsert forming so as to cover the peripheral edge portion of the through hole 3a. A rotational shaft insertion hole 7 is defined in the central portion of the hub 3 for receiving a rotational shaft 6 of a recording device (not shown). The inner surface of the hub defining the rotational shaft insertion hole 7 is provided with two reference surfaces 2a and a resilient member 2b to bias the rotational shaft 6 toward the reference surfaces 2a.
The inner peripheral edge of the recording disc 5 is secured to the top face of the annular supporting plate 3, and the upper hub 2' is secured to the outer peripheral edge of the hub 2 by means of a ultra sonic bonding so as to cover the inner peripheral portion of the recording disc 5. A ring like stopper 4 is secured on the lower surface of the annular supporting plate 3 by means of a suitable securing means. The stopper 4 has a diameter smaller than circular opening 1a defined in the central portion of the case 1 and the outer surface of the opening 1a can contact with the inner surface of the opening 1a so that the center of the recording disc 5 can be positioned at the center of the case 1.
When the recording disc 5 is mounted on the recording device, the recording disc 5 can be mounted on a position of the recording device with the rotational shaft 6 inserted in the rotational shaft insertion hole 7. Since the rotational shaft 6 is pressed onto the reference surfaces 2a of the hub by the force of the resilient member 2b, the recording disc 5 coupled to the hub 2 through the annular supporting plate 3 can be correctly centered. As the rotational shaft 6 is rotated under such a state as mentioned above, the recording disc 5 can be rotated in the case 1, whereby when a recording head is inserted in the case 1 through a head insertion hole, the recording head can slidably contact with the recording surface of the recording disc 5, so that recording and/or reproducing on the recording disc 5 can be made.
As is apparent from the explanation mentioned above, centering of the recording disc 5 can be made by the contact between the surface of the rotational shaft 6 and the reference surfaces 2a of the hub 2. Therefore, a high degree of accuracy of the reference surfaces 2a is required. Due to such construction, the rotational shaft insertion hole 7 can not be shaped into a circle since the reference surfaces are formed as flat planes, whereby in outserting the hub 2 with the annular supporting plate 3, the accuracy of the reference surfaces 2a is decreased due to different degrees of shrinkage of the various parts of the disc arrangement at the time of molding and cooling. Accordingly, the conventional recording disc mentioned above has such a drawback that it is difficult to increase the accuracy of the reference surfaces 2a, resulting in a lowering of the accuracy of centering of the recording disc 5 when the recording disc is mounted on the recording device.
The outer diameter of the top portion of the hub 2 is relatively smaller than the diameter of the opening 1b defined in the central portion of the top face of the case 1, and the top portion of the hub 2 and the end portion of the projected portion 3b are adapted to be exposed in the opening 1b. The projection 3b can rotate around the rotational shaft 6 when the rotational shaft 6 rotates, and a magnetic sensor 8 is provided above the rotation locus of the projection 3b. The magnetic sensor 8 is provided on the recording device side and is composed of a core 8a and a signal coil 8b and there is defined a clearance between the end face of the core 8a and projection 3b.
When the recording disc 5 and annular supporting plate 3 are rotated by the rotation of the rotational shaft 6, the projection 3b formed of a magnetic material and the core 8a of the magnetic sensor 8 come close to and go away from each other and the magnetic flux of the magnetic sensor changes. Accordingly, the signal coil 8b generates a signal to detect the position of the projection 3b i.e., the standard position of the recording disc 5 by the flux change.
In the conventional recording disc arrangement, as mentioned above, although the top end face of the hub 2 and the end face of the projection 3b, embedded in the hub 2, are adapted to be flush with each other, it is difficult to be flush with each other as desired due to an error of the size of the projection 3b and a further error of the size of the hub 2, at the time of the outsert molding, shortening the length of the projection 3b, whereby the projection 3b is positioned lower than the hub 2. If the projection 3b is too short, the projection is slanted by the pressure of the flow of the molten material and the projection 3b is displaced at the time of molding. As a result, it becomes difficult for the magnetic sensor 8 to detect the projection 3b. If the projection is too long, the projection is displaced by the pressure of the mold during molding. Thus, the magnetic sensor fails in sensing the projection 3b and the standard position of the recording disc 5 can not be correctly detected.